1. Field of the Invention
The invention pertains to the field of tracked vehicles. More particularly, the invention pertains to a steer drive with a differential for improved performance of a tracked vehicle under extreme low traction conditions.
2. Description of Related Art
Differential steering systems for tracked vehicles are well known. Such prior art track steering systems are often identified by such terms as “double differentials”, “steer drives”, and “cross-drive transmissions”, and these prior art steering systems are equally applicable to multi-wheeled off-road vehicles having no angularly adjustable turning axle. Of this prior art, the Gleasman steer drive disclosed in U.S. Pat. No. 4,776,235 has proven to be relatively inexpensive and remarkably effective in testing conducted on a full-terrain tracked vehicle (“FTV®”) built by Torvec, Inc. Using the Gleasman steer drive, the operator readily steers the FTV vehicle with a conventional steering wheel, as contrasted to the more conventional bulldozer-type drives with separate left and right control levers for each track, when traversing paved highways at highway speeds as well as when traversing off-road terrain.
Teachings of the prior art indicate that only some conventional form of unlimited-slip differential gearing may be used between the vehicle's engine and the track drives so as not to impair differential rotation of the drive axle shafts. All prior art differential steering drives for tracked vehicles use some conventional form of unlimited-slip differential gearing between the vehicle's engine and the track drives. Apparently, persons skilled in the art have believed that such a drive differential must be a differential lacking any limited-slip devices.
During extensive testing, a problem has been noticed when the FTV tracked vehicle is being turned on terrain that includes portions having unusually low traction. For instance, where one track of the vehicle is traversing extremely soft mud, that track can occasionally lose all traction and begin to “slip”. This is similar to the undesirable slipping that occurs in a truck with a conventional unlimited-slip differential, where one set of drive wheels begins to slip on mud, ice, or snow. When the FTV vehicle is turning and the entire track on one side of the vehicle loses traction, the turn is interrupted. In other types of differential drives if the track continues to slip when turning, the drive torque of the vehicle can be completely lost.
As explained in U.S. Pat. No. 4,776,235, the Gleasman steer drive is “no-slip” so long as the tracked vehicle is moving straight ahead or straight back and the steering wheel is held still by the operator. This no-slip condition results from the fact that the drives of both tracks are locked together when the steering worm/worm-wheel combination of the vehicle's steer drive is held motionless. Under this condition, the track drive shafts operate as if they were on straight axles without any separating differential. Nonetheless, when the steering motor drive of this prior art steer drive superimposes different track speeds for turning, the steering worm/worm-wheel combination begins to rotate, and this locked condition is lost. That is, the steer drive introduces differential action between the tracks, and when the drive shafts are differentiating, the loss of drive torque, i.e., slipping, may occur as it does in all conventional unlimited-slip differentials when one drive axle loses traction.
The sharpest turn that a conventional bulldozer-type drive, with separate left and right control levers for each track, can make is by braking one track while driving the other track, and this stresses the braked track considerably. Pivot turns using the Gleasman steer drive involve changing the direction of the vehicle with little or no translational movement of the pivot point at the center of the vehicle. Pivot turns can be power-assisted or powered totally by driving torque to be executed more rapidly. Since a vehicle is not using driving torque for forward or rearward movement when pivot turning occurs, driving torque is available for powering pivot turns. A slippage, similar to the turn slippage described previously, occurs during pivot turning, when one of the tracks is mired in a low-traction medium.
The interruption of steering or the loss of drive torque when one track slips, is endemic in all differential track drives and has apparently occurred in steer-driven tracked vehicles since their inception. As indicated in documentary information provided on television for the public with the consent of the United States government, this same slipping condition occurs with steer driven U.S. Army Abrams tanks. Abrams tanks also include a steering-wheel type drive in contrast to the more conventional bulldozer-type drives with separate left and right control levers for each track. While this condition is not sufficient to detract from the many advantages of tracked vehicles, it certainly has been a problem that has been plaguing tracked vehicles for a long time, and it occurs often enough in severe off-road terrain to justify correction. Avoidance of such undesirable steering problems is of particular importance for those few tracked vehicles that are capable of traveling at highway speeds.
There is a need in the art for a steer drive that prevents slippage when torque is suddenly reduced and that facilitates pivot turning for the tracked vehicle under extreme low traction conditions.